1. Field of the Invention
The present invention generally pertains to the field of radio transmission and, more particularly, point-to-point radio link communication systems using space-diversity reception techniques. Still more in particular, it concerns a method and a baseband combiner designed for symbol synchronization in receivers performing the combination of two more space-diversity signals.
2. Description of the Prior Art
Indeed, one of the main problems affecting the free-space links is the multipath phenomenon, also known as selective fading: the receive antenna can indeed receive, along with the desired signal, a delayed replica thereof, caused by the reflection of the transmitted signal against tropospheric layers or by reflection from orographic obstacles. In addition to this corruptive phenomenon is the so-called flat fading wherein the signal available at the receiving antenna is a combination of various signals not only delayed but also possibly attenuated.
Under particularly unfavorable conditions, the fading may even bring the radio system to an outage condition, thus making the received signal no longer intelligible.
A first possible, and widely used in practice, countermeasure is represented by the adoption of an adaptive equalizer inside the demodulation equipments. This solution may at times result in being inadequate in the case of radio links particularly long or installed under particularly unfavorable geographical conditions.
An alternative system remedy commonly in use provides for the use of space-diversity reception techniques using two or more suitably spaced antennas (in the following, by way of example, but not of limitation, two antennas will be considered) in reception. The space-diversity operating principle of the system just consists in sending the same information to the receiver by means of two distinct signals (one will be called “main” and the other will be called “diversity”). The effectiveness of this method depends on the fact that, if the antennas are sufficiently spaced out vertically, the received signals can be deemed to be uncorrelated and therefore it is extremely unlikely that both signals exhibit the same quality at the same time.
Two main methods of processing the pair of received signals are known: the selection (switching) and the combination. The switching is based upon the selection ideally at any time instant, of the best of the two signals through an appropriate criterion (typically, the evaluation of the Bit Error rate, in short BER).
The approach that is considered more effective is to process the two diversity signals by suitably combining them. The architecture which is often used in the case of baseband combiner, is the one wherein the main and diversity signals, suitably sampled, form the inputs of two Fractionally Spaced Equalizers (FSE) whose output is summed and form the result of the combination.
The clock signal recovery in equipments of the aforesaid type occurs according to the known Gardner algorithm [F. M. Gardner, “A BPSK/QPSK timing error detector for sampled receivers”, IEEE Transactions on Communications, vol. COM-34 No. 5, May 1986, pages 423–429] that utilizes the samples of the signal at T and at T/2 to provide an estimate that, after being properly filtered and integrated, then allows for recovering the correct sampling phase.
According to the known schematic of a baseband combiner of a radio link system with space-diversity reception, it is therefore a common practice to pick-up the signal at the input of the equalizer, be it of the main path rather than the diversity path, where the samples at T/2 are available. In principle, it would also be possible picking up the signal downstream the summation node where, on the other hand, both because of the demand to reduce the operating frequency of the upstream equalizers and because of the fact that the downstream decision device inherently requires the signal samples at T only, the samples at T/2 which, as said above, are essential to the Garden algorithm for the clock recovery, are not reasonably made available.
In the receive equipments having an architecture like the one described above, in order to realize the symbol synchronization in an effective manner, from which the clock signal to be utilized by the entire digital apparatus is also derived, it is then necessary to perform the clock recovery by means of a sole circuit.
In principle, it would be enough to previously fix indifferently the main or diversity signal to be inputted to the clock recovery circuit. However, this may be not enough to optimize the performances since the choice of which of the signals should be used for the clock signal recovery is decisive. Indeed, the channel conditions could particularly hinder or penalize the signal selected for synchronization with respect to the other one: in such a case the effectiveness of the whole combination, which, on the other hand should have had a different outcome if the clock recovery were caused by the less corrupted signal, would be impaired even also irremediably.